Blindness and impaired vision arising from retinal disease or damage are health problems with substantial human and economic costs. Recent advances in stem cell research suggest that retinal repair in humans will be feasible in the foreseeable future. However our understanding of retinal cell biology and development remains incomplete, limiting rational design of repair strategies. Cellular complexity is a key feature of the mammalian central nervous system, including the retina. The retina contains more than 50 different types of neurons, based on morphology, neurotransmitters, and other molecular markers. Cascades of transcription factors and various signaling pathways have been implicated in retinal cell type determination and the generation of cellular diversity in the retin. Recent studies have also implicated post-transcriptional regulation by microRNAs in the control of retinal cell identity. We have identified two related miRNAs that alter retinal development and promote amacrine interneuron formation, at the expense of other retinal cell types, when ectopically expressed in the developing mouse retina. We have used Argonaute PAR-CLIP to identify endogenous target mRNAs for these and other miRNAs in the neonatal mouse retina. Here we propose to determine the requirements for these miRNAs in amacrine cell formation, using CRISPR technology in retinas. We also plan to analyze a candidate target gene for its role in the regulation of retinal development, and we propose to analyze changes in mRNA expression to identify molecular and cellular pathways in the retina that are affected by these miRNAs. Finally, we propose to investigate the scope of miRNA regulation in retinal cells expressing the Ptf1a transcription factor, which is required for amacrine and horizontal cell formation. These studies will provide insight into the molecular mechanisms that control development and cell fate determination in the mammalian retina. They are expected to provide information that will contribute to new strategies to repair retinal tissue.